Stainless steel alloy and apparatus for converting hydrocarbons



F. J. SHORTSLEEVE ET AL 2, 050 STAINLESS STEEL ALLOY AND APPARATUS FOROCARBONS March 2, 1954 CONVERTING HYDR Filed March 31, 1950 '3Sheets-Sheet 1 L y?! I v w vwv vvw/vvv 35 3O Z5 20 W777; CHROM/UM OAYAVAY AYV R 4 INVENTORS:

Franc/ls J Shorfs/eeve Morris Nich lson Ym 7% .dTTO/PNEY March 1954 F.J. SHORTSLEEVE El AL 2,671,050

STAINLESS STEEL ALLOY AND APPARATUS FOR CONVERTING HYDROCARBONS 3Sheets-Sheet 2 Filed March 31, 1950 Quick? R E:

ON b 8 m3 2a m? mwmfiw an M PM W

nrfomvar M r 1954 F ORTSLEEVE ET AL 2,671 50 SH STAINLESS STEEL ALLOYAND APPARATUS FOR CONVERTING HYDROCARBONS Filed March 31, 1950 3Sheets-Sheet 3 W7." 7. NICKEL 0v 6 6 a IO :2 l4 l6 1s 26 W77 2,CHROM/UM- I INVENTORY: 27 Franc/L9 J Shorfs/eeve J? Morris E. NicgolsonEm A ATTORNEY Patented Mar. 2, 1954 STAINLESS STEEL ALLOY AND APPARATUSFOR CONVERTING HYDROCAR-BON S Francis J. Shortsleeve and Morris E.Nicholson, Chicago, Ill., assignors to Standard OilCompany, Chicago,

111., a corporation of Indiana Application March 31, 1950, Serial No.153,186

Claims.

This invention relates to improved oxidation and corrosion resistantchromium-nickel-silicon stainless steel alloys which are not susceptibleto embrittlement caused by the formation of sigma phase at hightemperatures. Further the invention relates to apparatus for convertinghydrocarbons which is adapted to resist the action of corrosivesubstances contained in the oil, and to withstand continued exposure tothe temperatures employed in heating the oil that is undergOlIlgtreatment without forming the embrittling sigma phase.

In order to increase the resistance of iron to corrosion and tooxidation at elevated temperatures, it is customary to add chromium.While excellent oxidation resistance results, the high temperaturestrength is not improved materially with increase in chromium content.This deficiency can be overcome by adding nickel but, for somecompositions, only at the expense of decreasing the resistance of thealloy to sulfur corrosion at the elevated temperatures. Both theiron-chromium alloys and the iron-chromiumnickel alloys in which thechromium exceeds about 20 Weight percent have the very seriousdisability that long exposure to temperatures of the order of 900 F. to1700 F. eventually results in a very great loss in creep strength andduetility-so great a loss that the metal often can be shattered with ablow from a hammer. This phenomenon has been described as embrittlementdue to the formation of the so-called sigma phasea complex compoundwhich is basically iron-chromium but may contain also some of the otheralloying elements found in stainless steels, such as, silicon,manganese, nickel, etc. It is known that addition of nickel, in moderateamounts, to chromium-iron alloys promotes the formation of the sigmaphase, which diiliculty has limited the use of the chromium-nickel-ironalloys in corrosive atmospheres when the metal is exposed to the rangeof temperature at which sigma forms, 1. e., 900-1700" F., depending uponthe particular composition. Above about 150 1700 F., depending upon theparticular composition, the sigma phase changes into either the ferritic(alpha) phase or the more desirable austenitic (gamma) phase and noembrittlement due to sigma exists.

As has been mentioned, the presence of nickel in some percentage rangereduces the resistance of the iron-chromium alloy to sulfur corrosion.It has been found that the addition of silicon overcomes this harmfuleffect of nickel, and also greases the oxidation and. ca burizaii nresist- 2 ance of the alloy. Besides the fact that silicon in amounts of3% (all compositions given herein are in weight percent) or more reducesthe workability of the alloy to the point that it is no longer practicalto make wrought articles, it has been believed that the addition ofsilicon in any amount was harmful in that it not only promoted theformation of sigma but also broadened the range of chromium contentsover which sigma could form. For this reason, in the chromiumnickelstainless steels, the amount of silicon usually has been kept as low asefiicient steel making methods permit; normally in typical steels suchas the 18Cr-8Ni and 25Cr-20Ni' compositions the silicon'conte'nt is lessthan 1%, for example, about 0.5%.

Other alloying elements that can be used to increase high temperaturestrength and increase oxidation and corrosion resistance are believed topromote sigma formation. Some of these elements are aluminum, columbium,titanium, molybdenum, manganese and cobalt.

In general the metallurgists in their search for alloys with highoxidation and corrosion resistance have avoided the elements thatwere-believed to be sigma phase promoters and have tried to stay out ofthe sigma-forming range of chromium contents. An example of such astainless steel, supposedly non-sigma-forming is the nominal 18Cr-8Ni.However, failures have occurred in such steels at temperatures in thesigma-forming range .that have not been explained because noconsideration was given to sigma formation. One of the objects of thisinvention is to develop a chromium-nickel-iron-silicon alloy that ishighly resistant to oxidation and corrosion at high temperatures andembrittlement. Another object is to develop a silicon containingchromium nickel stainless steel that does not form the sigma phase whensubjected to temperatures between 900-l700 F; for prolonged periods.Still another object. isto. provide a highly corrosion and oxidationresistant and non-sigma-forming alloy, containing at least about 1% andless than about 3% of silicon, that can be Wrought into such articles asfurnace tubes, jet turbine blades, etc. Yet another object is to providea stainless steel that is non-sigma forming and is an austenitic ormixed austeniticferritic type. f Since many hydrocarbon oils containsulfur compounds that decompose, while being heated for separation bydistillation or conversion into lower boiling materials, into highlycorrosive hy-; drogen. sulphide, it is an object of this inventionis notsubject to sigma J to develop an apparatus that is resistant to sulfurcompound attack at elevated temperatures and is not subject to hightemperature embrittlement from sigma-phase formation.

The invention by means of which these objects are achieved is based onthe discovery that a maximum effective-content otchromium existsatwhich, ifthe" nicheliccntent is; suitably adjusted, up to about 3% ofsilicon can be employed without introducing any susceptibility to formthe sigma phase. The invention comprises a nonsigma forming stainlesssteel containing-notmore than about 17.5 effective percent of' chromium,at least about 1% but less; than about 3.%: of. silicon, and an amountof nickel. which is: suitably adjusted at or above a hereinafterdescribed particular minimum requirementior theparticular chromium andsilicon content.

At an efiective chromium content oi 17.5%, the minimum nickel contentrequired to maintain a:- substantially austenitictype non-sigma-formingsteel will vary from at least about 1.0% at 1%: silicon content to atleast about 15%:at 3% silicon. content. If an alloy containing bothaustenite and ferrite is desired,- the nickel content must be reduced:to? less than at 1%. silicon and less than at 3% silicon and imorder toprevent sigma-iormation at these lower nickelcontents the: chromiumcontent also must bec'reduced as will be shown later; The remainder otthe alloy is principally iron and the-usual elements contained: incommercial stainless: steels,1 e:- g., manganese-which normally may bepresent in amounts lessthan" 2 carbon is preferably less than 011%: butmay be present in amounts up to 0.5 and steel making amounts oiphosphorus; sulfur, and miscellaneous elements such as nitrogen; cobalt,vanadium, columbium, titanium, etc.

The stainless steel compositions of our invention areillustrated in: thefollowing figures which form a part of: this specification.

Figures" 1. and 2 are plots on. triangular coordimates of. theiron-chromium-nickel system. with variousamounts of silicon present, and

Figure}; is a cartesian plot. showing the relationshi-p between nickeland chromium at various sihcon. contents.

The invention can. be better" understood by reference to Figure 1 whichshows on triangular coordinates the phase diagram ABCDHJE of the highvpurity irmi-chromiunr-nickel; system at- 1200 F. The high purityboundaries are based onithe best: results published and our own work.'Ihecompositions falling within the ABCDI-IJ portion of the ternarydiagram are not susceptible to sigma formation. This portion is dividedinto the'austenitic (7) phase area ABH, the ferritio (al phase area JCI)and the mixed austenitic plusferritic (7+0) phase area HBCJ; B is thegamma; triple point and C is the alpha triple point.

While for some purposes a non-sigma forming ferritic' steel may bedesirable, the austenitic and austenitic-ferritic steels have the mostdesirable high temperature properties. When an extremely tough alloythat is particularly resistant to oxidation at elevated temperatures isneeded, an alloy that is wholly austenitic or substantially austeniticis required. Our' invention is primarily concerned withiron-chromiumniokel-silicon alloys that are austenitic or mixedaustenitic-ferritic in nature;

In addition to the high purity alloy boundaries there-appear Figure 1theboundary lines sep- 4 arating the sigma and non-sigma forming regionsfor the iron-chromiurn-nickel alloys to which 1, 2 and 3% of siliconhave been added; these boundaries appear as the solid lines that divergefrom the line AB.

The dotted lines in Figure 1 represent the previouslyaccepted: positionsof the silicon addition phase boundariesof iron-chromium-nickel alloys.These boundaries were constructed as the best possible synthesis of theconclusions expressed in these articles: Schafmeister and Ergang,Archiv" fiir: das Eisenhiittenwesen, vol. 12, 459 (1939') Gowand Harder,Trans Amer Soc Metals, vol. 30,. 855 (1942); Payson and Savage, TransAmer: Son. Metals, vol. 39, 404. (1947).

In ohd'erto show the eiiect of silicon addition moreclearlmthe areaenclosed by the heavy lines WXYZ' has been replotted on a larger scalein Figure. 2. Our work indicates that when about 1% of silicon is added.to a high purity ironchromium-nickel alloy the gamma triple pointshimzto thepoint B at Ni=10-.O-% and the alpha shiitsto the point C atNi=3.5%= and Cr=l,4a5;%-. When about 2% of silicon are added the gammatriple: point shifts to the point B at Ni=12.587 and. the alpha. triplepoint shifts to the point. C? at Ni=4.0.%-. and Cr=12.5 At: a,silicorraddition of about 3% the gamma triplepoi-nt shiftsto B atNi=-1-5.0 and the alpha triple point shifts to C at Ni=4.5-% and.Cr=10.5;%. The lines B 8 B 0 andB3C which form the phase bound.- arybetween the sigma and non-sigma forming (1+ area and the a+- +r area isa. straight line.

Figures 1 and; 2 indicate that in so far as. experimental accuracypermitsin the location of the line, the phase boundary between theaustenitic. area and the austenitic plus sigma area: for the. siliconaddition alloy-up? to about 3% of silicon addition-coincides. with thehigh purity alloy boundary at about 11.5%. chromium content. (Theexperimental error in the determination of this position. is. believedto be less; than about 0.5% absolute.)

Ihe short lines pointing toward the. iron scale of the chart indicatethe. approximate position of the boundary between the gamma. and gammaplus alpha areas and the gamma plus alpha. and the alpha areasrespectively.

While our experiments have not established with certainty the point,where the line AB be gins to bend significantly, it appears: that thedeparture. from a straight line, or nearly straight, begins tobeapprcciable at about 20% nickel content.

The dotted. lines in Figure 2 illustrate the shape and location of thesilicon addition boundaries as previously accepted. Lines M N P M N Pand M3N P correspond to silicon contents of 1%, 2% and 3% respectively.

Our discovery explains the heretofore unexplainable failures of thenominal 18Cr-8Ni steels inprolonged service. at temperatures in therange of 9410-4500 F. These alloys are sigma-forming because the nickelcontent is too low to overcome the sigma-forming tendency of the about0.5% of silicon usually present in the commercial puritystainlesssteels. Figure 2 indicates that about 1% more nickel atabout 17.5%chromium, instead of the 18-20% specification, would render the nominal18-8 steels non-sigma forming.

Thusour invention comprises iron-chromiumnickel .alloys containing fromat least about 1%- to not more than3% V of silicon which are not sus--ceptible tosigma phase formation and which confiist of the austenitic ormixed austenitic-ferriticphases. The area bounded-by the lines MNPTBArepresents the composition of our stainless steel alloy at siliconcontents ranging from at least about 1% to not more than about 3%; theline MNP represents the minimum amount of chromium content and the lineABT represents the maximum amount of chromium content in ourcomposition; the line ABT represents the minimum amount of nickel neededto overcome the sigma forming tendency of the chromium and silicon butthe nickel content can be greater than this minimum but preferablyshould be less than about 20 (Of course when We speak of the lines MNPand ABT, we mean to imply the line with the superscript corresponding tothe desired silicon content.)

In order to show still more clearly the variation of the maximumchromium and minimum nickel content of the iron-chromium-nickel alloywith varying silicon, the phase boundary lines ABC have been placed on acartesian plot of nickel vs. chromium which is illustrated as Figure 3.Also shown in Figure 3 are the previously accepted silicon additionboundary lines MNP. The letters have the same significance in all threefigures. 'The lines PT, PT and P T represent a portion of the previouslyaccepted boundary between the ferritic area and the ferritic plus sigmaphase area; T represents the junction of this previously acceptedboundary and the line BC of our phase diagram. The line T T T indicatesthe manner in which this intersection point shifts its location withchanges in silicon content from 1% to 3%.

Although the line T T T has been drawn as a straight line, it is moreprobably somewhat curved. However, the deviation from the straight linerelationship is believed to be within the experimental error. When T T Tis considered to be a straight line, the location of any point on theline can be calculated using the following relationship (for conveniencethe following symbols have been adopted for use in equations: Si=wt. percent silicon; Ni=wt. per cent nickel; Cr=wt. per cent efiectivechromium) Cr=l7.0-1.7Si and Ni=4.5+0.7Si. The relationship betweennickel and chromium for a T T T line alloy is approximately,Ni=10.00.3Cr.

The silicon addition lines, in Figure 3, AB C AB C and A38 0 can bedivided into two segments for easier analysis. Line AB is relativelystraight at an effective chromium content of 17.5% from a nickel contentof about 8.5% to about 20%. The gamma triple points represent theminimum content of nickel, at the particular silicon percentage,necessary to maintain a nonsigma forming composition; this amount is thegamma triple point nickel content. The minimum nickel percentage can becalculated for any variation of silicon between about 1% and 3% by theformula: minimum Ni=7.5+2.5Si, when the effective chromium content isabout 17.5%.

The boundaries B C B 0 B 0 form a family of straight lines whose slopemay be determined from the percentage of silicon present, as follows:

Figure 3 indicates that the amount of nickel 7.

present can be greater thanthe calculated mini- 'mum amount withoutadversely affecting the sigma susceptibility; however, it is preferredto maintain'the nickel content at less than about 20%.

In much the same manner the previously accepted silicon addition lines,in Figure 3, MW P M N P and M N P can be divided into two segments foranalysis; each of these segments can be considered as approximately astraight line. It is estimated that line LPN! is approximately astraight line at a constant chromium content from a nickel content ofabout 13% to about 20%. A generalized relationship has been determinedbetween the previously accepted maximum amount of chromium that can betolerated in the composition and the silicon content, and between theminimum amount of nickel previously thought necessary to maintain anon-sigma forming composition at the particular silicon content, asfollows: maximum Cr=17.52.0Si and minimum Ni=9.0+2.0Si.

The previously accepted boundaries N P N P and N P forms a family ofapproximately straight lines whose general slope is=0.2Si. Therelationship between the minimum amount of nickel previously thought tobe needed to maintain a non-sigma forming alloy at a particular chromiumand silicon content is,

The PT lines shown in Figure 3 represent the previously acceptedboundary between the ferritic area and the ferritic plus sigma area.These lines form a family whose relationship can be generalized as:

This line is the low chromium-low nickel section of the polygonal areawhich represents the stainless steel composition of our invention.

Thus our invention comprises a stainless steel (of commercial purity inrespect to elements other than chromium, nickel and silicon) whichcontains, at least about 1% but less than 3% of silicon, an effectiveamount of chromium of not more than about 17.5% and a lower limitdependent on the silicon content, at least'a mini mum amount of nickelwhich amount varies with silicon and chromium content (which minimumamount of nickel, and, lower limit of chromium, can be determined byreference to the area enclosed by the letters ABTPNM at the desiredsilicon and chromium content) and the remainder principally iron. Figure3 indicates that at about 2% silicon content a non-sigma formingcomposition can be produced which may contain from at least 13.5% toabout 17.5 eiiective percent of chromium, from at least about 12.5% to20% of nickel and the remainder principally iron, which composition issubstantially all austenitic phase; also a composition can be producedwhich may contain from at least about 11.5% to about 15% of effectivechromium at a nickel content of about 8%, from at least 13.5% to 17.5%of efiecti've chromium at a nickel content of 12.5% and the remainderprincipally iron, which composition is a mixed austenitic-ferritic phasealloy;

The composition of our stainless steel alloy can be more accuratelydefined by mathematically describing the perimeter of the composition as"1 cente iie 'waQhmm -u1n-v E m;- en ice. lcr-si co ten ircm z stabout1%. o than about 35711. e mb sition. oi our nyention may be consideredas an area obtained by plotting thechromium and nickel contents of the.phase boundary between the sigma containing: and; the sigma-free 'y and'Y.+G- phases for the phase diag am as. t is. ow lo at d. by our workand the previously accepted location, The upperlimit on nickel contentour composition about 20% which limit is imposed by the. fact that:nickel contents above 20% make the stainlessstccl unduly: susceptible tohigh temperature corrosion. The silicon of our alloy should beatleastabout 1% in order toobtain the benefit oiincreased oxidationresistance but should be held below about 3% in order to permit themanufacture of; wrought articles. The area med by the invention isbounded: the maxiamount. of nickel is less than about 20 the maximumeffective amount of chromium; is about.17.5% when the amount of nickelis at least, Ni =,'7.5+2.5si; between the limits (Cr: 17.5, Ni=7.5+2.5S'1) m=(1.0+2.0Si) /(4.5+2.0Si)

between h im ts.

(C1=17.0-=.1.7Sl, Ni=4.5+0.7Si)

and (Cr=17.5-3.0Si, Ni=4.0+2.0Si), the effective chromium content andthe minimum nickel content vary in accordance with equation:

Cr=21.5Si-Ni between the limits,

(Cr=l'7.5-3.0Si, Ni=4.0.+2.0Si)

and (Cr=17.5-2.0Si, Ni=9.0+2.0Si), the effective chromium content andthe minimum nickel content vary in accordance with the equation:=19.0.-5.5S i+0.2Si(Ni) between the limits (Qr=17.52.0Si, Ni=9.0+2 .0Si)and Ni=about goathe effective chromium content is at least aboutCr=1'7.5-2.0S i; and the remainder of the alloy principally iron. As hasbeen pointfld out elsewhere in this specification; the equations of thesides of the polygon that bounds our compo? cities. and he cal la e prip ry breakp s are general z r m a t p ific l c on tents and allowancemust be made for some deviation from the actu l o by t llowable. exprimen al r o in i type Qt o Although we speak of at leastabout 1% ofsilicQ we realize that silicon may be present in comm ci l puritystainle ssteels. n amount to. about 0.7% but silicon is considered to bean adde l o m nt n se n o n greater than 0.7%. When. We, Speak ofsilicon content, we mean the total silicon present as def termined byanalysis.

Reference to Figure 1 will show that the above compositions vary inphase content from all gamma, phase to a mixture of alpha and gammaphases. For some purposes an extremely tough alloy that is particularlyresistant to oxidation at. elevated temperatures is desired; for theserequirements an alloy that is entirely austenitic (gamma phase) orissubstantially allaustenitic s ce se r- Fer u ses the. s icoacou cnt sld a ea t bou .7 in order t set the ben fit: 7. n r a ed oxidation. esisce bu sno d ekep below abou 3% in or er t erm t the alloys tobemanufactured into wroughtafiticles; the effective chromium contentshould be kept aboveabout 10% order to afford sufficient oxidationresistance at temperatures in the range 1000,-.15 00 F. and must be keptbelow about 17.5% inorder to avoid sigma formation; the nickel contentmust be high enough to avoid sigma formation and improve. workabilitybut should be held below about 20% in ordertoprevent making thealloyunduly susceptible. to. high temperature corrosion. The minimumamount of nickel that is necessary in an alloy that is nonsigma formingand is all or substantially all austenitic is that defined by theformula;

The susceptibility of a stainless steel alloy to form sigmav isprimarily determined by the amount of chromium in the solid solutionphase. Itis well known in the art that the carbon pres,- ent instainless steels of commercial purityfmay combine with chromium to forma carbide which may precipitate out of the solid solution and therebyreduce the amount of chromium availe able in th solid solution. Thiscarbide formation makes the effective amount of chromium lower than isshown as present by the, chemical analysis of the alloy. The net effectis to cause the alloy to behave. as though it has a lower chromiumcontent and is less susceptible to sigma formation. Since it isestimated that 0.1% of carbon will remove on the order of 1%v of 1chromiumfrom solid solution, many borderline stainless steel ay berendered non-sigma forming by the presence of carbon. Our work has takeninto account the effect of carbon and we prefer to speak of theefiective chromium content and thereby eliminate the uncertaintyresulting from varying carbon content.

While we recognize the beneficial efi'ects of carbon on sigma-formingtendencies, we prefer to keep the carbon content of our compositions lowbecause of the deleterious effect of carbon on other properties. Thepresence of the carbides generally is undesirable except where wearresistance is wanted. Their presence may seriously impair the ductilityand impact resistance cf the stainless steel.

It is known that, in general, manganese, 00-. belt and copper act muchlike nickel their effects on the sigma transformation; however, it isbelieved that their effect is somewhat less than that of a similaramount of nickel. While stainless steels normally contain less than l%of. man ane mum d contain a uc as without being classed as manganesesteels. Al} though we prefer to use nickel as our alloying agent toobtain workability, we could decrease the amount of nickel present andmake up the necessary percentage by adding suitable amounts ofmanganese, cobalt, copper or combinations thereof to the composition'Thus when we re,- fer to nickel, we means to include those compositionswherein manganese, cobalt or copper are added to the chromiumenickelstainless steel to replace some of the nickel in our composition.

Our experiments indicate that the presence of; minor amounts of theelements usually found in commercial purity stainless steels will notaffect the i m o ming op r e o r Pre erred. compositions. Thes el nt abe etc. Nitrogen is known phosphcr s, an um to p mo eusteni c ph se.iqrmatiqa so that addition of minor amounts may be desirable. The termssubstantially all iron or principally iron are intended to include theseminor elements as well as the usual content of manganese inchromium-nickel stainless steels.

The following alloy, after 500 hours exposure to a temperature of 1200"F., was found to be a mixture of gamma and alpha phases: chromium,14.77%; nickel, 7.87%; silicon, 0.90%; carbon, 0.06%; manganese, 0.78%;molybdenum, 0.03%; sulfur, 0.010% phosphorus, 0.022%; and the remainderiron. Other compositions which do not form the sigma phase when exposedfor long periods to temperatures in the range of 900-1700 F. are:

Percent P the remainder of the alloy being iron and steelmaking amountsof the other elements, such as given for the complete composition above.

Since our composition has excellent oxidation and corrosion resistanceas well as high strength at elevated temperatures and is not susceptibleto sigma embrittlement at these temperatures, it can be used anywherethat such properties are desirable. A few of these fields are: tubes inhigh pressure steam boilers, turbosuperchargers, et engine parts, gasturbines, chemical roasting equipment, high temperature chemical processequipment, etc. A principal field of use for our composition is in theheating of hy drocarbon materials to temperatures above 800 F. and inthe thermal or catalytic cracking of hydrocarbons to produce gasoline.The heating for such conversions is ordinarily carried out in tubes orcoils which may be subjected to oxidizing conditions on the outside andsimultaneously to the action of sulfur compounds associated with thematerial being cracked within the tubes. Our alloys are admirably suitedto the fabrication of tubes and containers for such high temperatureoperations. The heating of hydrocarbons is a particularly suitable fieldfor the use of our alloy because of the variety of stocks and operatingconditions which must be met. Therefore We illustrate the use of ouralloy in this field by several operating examples.

In a coil only type thermal cracking process, the feed to the processoften is a high sulfur gas oil, for example, a 25 API West Texas gas oilcontaining 2.0% total sulfur. The oil is heated at a pressure of about400 p. s. i. g. to a transfer line temperature of about 925 F. before itleaves the furnace and passes to a coldoil quench which stops thecracking reaction and prevents excessive formation of unwanted tarrymaterial. During its passage through the tubes of the furnace the sulfurcompounds in the oil decompose to form hydrogen sulfide, which isextremely corrosive at high temperatures; simul taneously the outersurface of the tube is exposed to the radiant heat of the burners andthe oxidizing action of the hot combustion gasesoften the tubes operateat red heat. Experience shows that a relatively high chromium-nickelalloy is needed to overcome the simultaneous corrosive and oxidizingconditions; however, these alloys have been subject to embrittlementfailures and so are not used by many refiners who employ instead lowchromium alloy tubes and accept a shorter tube life. It is ordinarypractice to operate this type of cracking process for run lengths ofabout 2000 hours, which may be sufficient time to form harmful amountsof sigma if the alloy is susceptible to sigma formation.

Another type of thermal cracking operation is the so-called tube andtank process wherein the feed is heated to 8'75-900 F. and about 500 p.s. i. g. before the oil is transferred to an insulated drum, where it isheld and allowed to continue the cracking reaction until the desiredamount of conversion has taken place. The heavy tarry materialwhich iswithdrawn from the reactor may have a sulfur content of between 3 and 4%so that a highly corrosion-resistant material is required. As theliquids in the vessel may be at a temperature of above 800 F. and therun lengths for this type of operation may reach 3000 hours, it isprobable that materials that are susceptible to the formation of sigmawill form such in the furnace tubes and may do so in the reaction drum.

In a catalytic cracking process, the principal duty of the furnace is toheat the feed very rapidly to a temperature of about 900 F. so thatlittle cracking takes place, as it is desired that the cracking takeplace in the presence of the catalyst, away from the furnace. The mosteconomical operation of catalytic cracking processes requires extremelylong run lengths of the order of 6000-8000 hours, and a trouble-freefurnace is essential to such run lengths.

While we have listed many examples where our compositions could be usedadvantageously, we do not intend that this list be considered exhaustiveand we intend to include any and all uses that can be made of thisalloy.

We claim:

1. An apparatus for heating hydrocarbons comprising a furnace, tubespositioned within said furnace and means for heating said tubes totemperatures on the order of about 900 to 1700 F., said tubes beingformed of an austenitic-type stainless steel consisting of, on a weightbasis, silicon: between at least about 1% and not more than 3%, nickel:between at least equal to the sum of 7.5+2.5 per cent silicon and about20%,

chromium: about 17.5%, and the remainder substantially all iron, whichstainless steel is characterized by freedom from sigma phase afterprolonged exposure to temperatures on the order of about 900 to 1700 F.

2. The apparatus of claim 1 wherein the tubes are formed of acomposition consisting of chromium, 17%; silicon, 2.0%; nickel, 15.5%,and the remainder substantially all iron.

3. The apparatus of claim 1 wherein the tubes are formed of acomposition consisting of chromium, 17%; silicon, 2.8%; nickel, 14.5%and the remainder substantially all iron.

4. An apparatus for heating hydrocarbons to a cracking temperaturecomprising a furnace, a heating coil positioned within said furnace andmeans for heating said coil to temperatures within the range of about900 and 1700 R, which coil is formed of an austenitic-type stainlesssteel consisting of, on a weight basis, silicon: between at least about1 and not more than 8%, nickel: between at least the sum of 7.5+2.5 percent silicon and about 20%, chromium in substeiitielllfir an man, whichsteel iiecharaeteri'zd by the ability ti) W'ith's'fi'alnd prblofigedexposure tetemp'erahires of between about 900 'a'nd'1700 F. Withoutforming sigma phases 5. An apparatus fqrheating hydrocarbons 00-1prising '2, 'furnece tubes positioned Within said furnace and means forheating said tubes to atiiiperature on the order of about 900 to about1700" R, which tubes are ffqrmed'of an austenitictype Stainless steelconsisting of, on a weight basis, silicon about 2%, nickel between atleast 12,576 and about 20%, chromium about 17 .5% and the remaindersubstantially all iron, "which tubes *aifle cha'recfiermzd by freedomfrom sigma phase after prolonged. exposure'to 'temperatures en-theofder'0f900'to 1700 F.

FRANCIS J. SHORTSLEEVE. MORRIS E. NICHOLSON.

bate Q 'Niinib'er 7 June 2'7, 1922 OTHER RE."FIEHREIENCES Stainless Ironand Steel, pages 396 anh 580'; edited by Monypenny. Published in 1931 byChapman andyHall, London, England.

Archer, Refiner and Natural Gasoline Manufacturer, v01. -20,-pages262-268 and 281, July Morton, The Oil and Gas Jbiirne of June26,1941, pags 60-63.

1. AN APPARATUS FOR HEATING HYDROCARBON COMPRISING A FURNACE, TUBESPOSITIONED WITHIN SAID FURNACE AND MEANS FOR HEATING SAID TUBES TOTEMPERATURE ON THE ORDER OF ABOUT 900* TO 1700* F., SAID TUBES BEINGFORMED OF AN AUSTENITIC-TYPE STAINLESS STEEL CONSISTING OF, ON A WEIGHTBASIS, SILICON: BETWEEN AT LEAST ABOUT 1% AND NOT MORE THAN 3%, NICKEL:BETWEEN AT LEAST EQUAL TO THE SUM OF 7.5+2.5X PER CENT SILICON AND ABOUT20%, CHROMIUM: ABOUT 17.5%, AND THE REMAINDER SUBSTANTIALLY ALL IRON,WHICH STAINLESS STEEL IS CHARACTERIZED BY FREEDOM FROM SIGMA PHASE AFTERPROLONGED EXPOSURE TO TEMPERATURES ON THE ORDER OF ABOUT 900* TO 1700*F.